If you have an older model, you could just upgrade the motor to the beltless Mk2 version. I just saw one on Ebay for US$289. If you really want to upgrade your course computer, I just saw an S1 for ~$200 with 0 bids and ending in 14 hours.
They run well, far easier to install and maintain. I had one for 4 years and never changed internal belt. The look seems beter too. But in fact, if your actual AP works fine, I don't really think it will be an upgrade, since the "piloting" function has no significant improvement.